build: NuGet cache mount + buildx registry cache

Mirror the cache infra added to nas-auth (commit a1ecba3) and
obsidian-mcp. Same pattern: --mount=type=cache for restore+publish
in Dockerfile, registry cache mode=max in workflow.

Dockerfile

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1.6
 # ── Stage 1: build ──────────────────────────────────────────────
 FROM mcr.microsoft.com/dotnet/sdk:10.0 AS builder
 
@@ -5,11 +6,13 @@ WORKDIR /src
 
 # 先复制 csproj，单独 restore（利用层缓存）
 COPY gitea-mcp.csproj .
-RUN dotnet restore gitea-mcp.csproj
+RUN --mount=type=cache,target=/root/.nuget/packages \
+    dotnet restore gitea-mcp.csproj
 
 # 复制剩余源码并发布
 COPY . .
-RUN dotnet publish gitea-mcp.csproj \
+RUN --mount=type=cache,target=/root/.nuget/packages \
+    dotnet publish gitea-mcp.csproj \
     -c Release \
     -o /app/publish \
     --no-restore